1. Field of the Invention
This invention relates to a display device, especially to an electroluminescent display device.
2. Description of the Related Art
An EL display device with an electroluminescent (referred to as EL hereinafter) element has been gathering attention as a display device substituting a CRT or an LCD. The development effort for the EL display device with a thin film transistor (referred to as TFT hereinafter) as a switching element for driving the EL element has been made accordingly.
FIG. 9 is a plan view of a pixel portion of an organic EL display device. FIG. 10A is a cross-sectional view of the display device along with the Axe2x80x94A line, and FIG. 10B is a cross-sectional view of the display device along with the Bxe2x80x94B line shown in FIG. 9.
A pixel portion 115 is formed in the area surrounded with a gate signal line 51 and a drain signal line 52 as shown in FIGS. 9, 10A and 10B. The pixel portions are disposed in a matrix configuration.
An organic EL element 60, which is a light emitting element, a switching TFT 30 for controlling the timing when to supply electric current to the organic EL element 60, a driving TFT 40 for supplying electric current to the organic EL element 60, and a storage capacitance element are disposed in the pixel portion 115. The organic El element 60 is configured from an anode layer 61, an emissive element layer made of light emitting material, and a cathode layer 65.
The first TFT 30, the TFT used for switching, is disposed near the crossing of the two signal lines 51, 52. The source 33s of the TFT 30 works also as a capacitance electrode 55, which forms capacitance with a storage capacitance electrode line 54. The source 33s is also connected to the gate 41 of the second TFT 40, the TFT for driving the EL element. The source 43s of the second TFT 40 is connected to the anode layer 61 of the organic El element 60, and the drain 43d of the second TFT 40 is connected to a driving source line 53, which supplies electric current to the organic EL element 60.
The storage capacitance electrode line 54 is disposed parallel to the gate signal line 51. The storage capacitance electrode line 54 is made of chrome and forms capacitance by accumulating electric charge between itself and the capacitance electrode 55 connected to the source 33s of the TFT through a gate insulating film 12. This storage capacitance 56 is disposed in order to keep the voltage applied to the gate electrode 41 of the second TFT 40.
The organic EL display device is configured by forming the TFT and the organic EL element consecutively on a substrate 10, which is made of a glass, a synthetic resin, a conductive material, or a semiconductor, as shown in FIGS. 10A and 10B. When the conductive substrate or the semiconductor substrate is used as the substrate 10, an insulating film made of SiO2 or SiN should be first disposed on the substrate. Then, the first and second TFTs and the organic EL element are formed. Both TFTs should have a top-gate configuration, where the gate electrode is located above an active layer with the gate insulating film between them.
Next, the first TFT 30 for switching will be explained.
An amorphous silicon film (referred to as a-Si film hereinafter) is formed through a CVD method on the insulating substrate 10, which is made of quartz glass or non-alkaline glass as shown in FIG. 10A. The a-Si film is irradiated by a laser beam for recrystallization from melt, forming a poly-crystalline silicon film (referred to as a p-Si film, hereinafter). This functions as the active layer 33. Single layer or multiple layers of a SiO2 film and a SiN film are formed on the p-Si film as the gate insulating film 12, on which the gate signal line 51, also working as the gate electrode 31, made of a metal with a high-melting point such as Cr and Mo as well as the drain signal line 52 made of Al are disposed.
A SiO2 film, a SiN film and a SiO2 film are sequentially disposed to form an interlayer insulating film 15 on the entire surface of the gate insulating film 32 and the active layer 33. A drain electrode 36, which is formed by filling a contact hole formed at the location corresponding to the drain 33d with a metal such as Al, is disposed, and a planarization film 17 made of organic resin for flattening the surface is formed on the entire surface.
Next, the second TFT 40, the TFT for driving the organic El element will be explained. An a-Si film is formed on the insulating substrate 10. The a-Si film is irradiated by a laser beam for forming a poly-crystalline silicon film functioning as the active layer 43. The gate insulating film 12, and the gate electrode 41 made of a metal with a high-melting point such as Cr and Mo are deposited on the active layer 43. A channel 43c is formed in the active layer 43. A source 43s and a drain 43d are also formed at both sides of the channel 43c. A SiO2 film, a SiN film and a SiO2 film are sequentially disposed to form the interlayer insulating film 15 on the entire surface of the gate insulating film 12 and the active layer 43. The driving source line 53, which is connected to the driving source by filling a contact hole formed at the location corresponding to the drain 43d with a metal such as Al, is disposed. Furthermore, the planarization film 17 made of organic resin for flattening the surface is formed on the entire surface. A contact hole is formed in the planarization film 17 at the location corresponding to the source 43s. The anode layer 61 of the organic El element, which is a transparent electrode made of ITO making contact with the source 43s through the contact hole described above, is formed on the planarization film 17. The anode layer 61 is disposed forming an island for each of the pixel portions.
The organic EL element 60 includes the anode layer 61, a hole transportation layer 62 having a first hole transportation layer made of MTDATA (4,4-bis (3-mathylphenylphenylamino) biphenyl) and a second hole transportation layer made of TPD (4,4,4-tris (3-methylphenylphenylamino) triphenylanine), an emissive layer 63 made of Bebq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium) including quinacridone derivative, an electron transportation layer 64 made of Bebq2, and the cathode layer 65 made of either magnesium-indium alloy, aluminum, or aluminum alloy.
The holes inputted from the anode layer 61 and the electrons inputted from the cathode layer 65 are re-combined in the emissive layer of the organic EL element 60, activating organic molecules of the emissive layer. When the activated molecules are deactivated, light is emitted from the emissive layer, and passes through the transparent anode layer 61 and the transparent insulating substrate to escape outside the display devie as illuminating light. The above technology is described in, for example, Japanese Laid-Open Patent Publication No. H-11 283182.
The cathode layer 65 is disposed covering the entire pixel portion 115 in the above organic EL display device. FIG. 11 shows the layout of one of the pixels of the display device. The pixel portion 200, which is the same as the pixel portion 115 of FIG. 9, has an organic emissive portion 201 that includes the organic EL element 60 and a circuit portion 202 that includes the switching and driving TFTs. The cathode layer 1C of the organic El element is disposed on the entire pixel portion 200.
The wiring materials such as aluminum used for the cathode layer is only utilized as the cathode layer, and they are not used for other wiring in the conventional organic EL display device. Therefore, the open aperture of this display device decreases when new signal lines and new source lines are formed in the display panel, leading to lowered luminescence of the organic EL display device.
The invention provide a display device having a plurality of pixels. Each of the pixels includes an electroluminescent element having an anode layer, a cathode layer and an emissive layer disposed between the anode layer and the cathode layer, a driving transistor for driving the electroluminescent element, and a cathode level wiring layer disposed above the driving transistor. The cathode layer is a part of the cathode level wiring layer. The device also includes a device wiring layer that is a part of the cathode level wiring layer and is electrically insulated from the cathode layer.
The invention also provides a display device including a plurality of pixels. Each of the pixels includes an electroluminescent element having an anode layer, a cathode layer and an emissive layer disposed between the anode layer and the cathode layer, a driving transistor for driving the electroluminescent element, a peripheral driving circuit for driving the driving transistor, and a cathode level wiring layer disposed above the driving transistor. The cathode layer is a part of the cathode level wiring layer. The device also includes a peripheral wiring layer that is a part of the cathode level wiring layer and is electrically insulated from the cathode layer. The peripheral wiring layer is connected to the peripheral driving circuit.